1. Field of the Invention
The present invention relates generally to micro electromechanical systems (MEMS) and, more particularly, to diffractive light modulators.
2. Description of the Background Art
A MEMS or micro electromechanical (MEM) device typically includes micromechanical structures or light modulators that may be actuated using electrical signals. The light modulators may comprise, for example, Grating Light Valve™ (GLV™) light modulators available from Silicon Light Machines, Sunnyvale, Calif. (GLV™ and Grating Light Valve™ are trademarks of Silicon Light Machines). A light modulator may include an array of moveable structures referred to as “ribbons.” Light modulators may be used in various applications, including video, printing, optical switching, and maskless lithography, as just a few general examples.
FIG. 1 illustrates a conventional diffracting element surface structure 100 that can be part of a light modulating device. This structure includes two surface levels, with one generally movable and one generally fixed. These are labeled as “upper” 102 and “lower” 104 reflecting surfaces in FIG. 1. Each of these surfaces has substantially equal area and equal reflectivity properties. Also, the height difference between each surface (i.e., in the direction of the light to be modulated) is changed to modulate the relative phase difference for light reflected from each surface. If, upon reflection, the light from both surfaces is “in phase,” then the 0th order light reflection is effectively maximized. If, upon reflection, the light from both surfaces is “out of phase,” then the 0th order light reflection is effectively minimized. To minimize the 0th order reflection, the height difference can be 1λ/4, 3λ/4, 5λ/4, 7λ/4 or 9λ/4, etc., where λ is the wavelength of the incident light. To maximize the 0th order reflection, the height difference can be 2λ/4, 4λ/4, 6λ/4, 8λ/4 or 10λ/4 etc. These maximum and minimum 0th order reflectivities may only be realized with an optical system that can appropriately discriminate between various diffraction orders. Typically, within one modulating element, the spatial frequency of these surfaces must be greater than that of the modulating element. In many implementations, it is at least twice as much. For the example case illustrated in FIG. 1, a “pixel” can include at least 2 upper surface reflecting features and 2 lower reflecting surfaces. Similarly, optical systems can be built which may select diffracted light, and the diffracted light can be modulated from about a maximum to about a minimum value in the same manner.
FIG. 2 is an illustration of a conventional light modulator in both cross section and top view. This represents a very particular example implementation where “ribbons” are used as modulator elements. In this example, the ribbons 202 and gaps 204 are the same width. Also, the ribbons 202 and gaps 204 are typically a uniform width for their entire length. Further, the gaps 204 and ribbons 202 are typically covered with the same reflecting material 206, which can be Aluminum or other reflective-material. The height difference, as related to the cross section diagram, is initially controlled by the film thickness choice. The height difference (and hence the reflective or diffractive condition of the modulator element) can be changed by controllably deflecting the ribbons 202 by up to about λ/4, where λ is the wavelength of the incident light. Here, It is assumed that the light is at or near normal incidence, i.e., perpendicular to the plane of the device. Accordingly, in the cross section diagram, the light direction would be from the top of the page to the bottom of the page. In the top view diagram, the light direction would be onto the page in a direction normal to the page.
FIG. 3 shows an illumination profile 301 for a conventional diffractive element 300. When the ribbons 302 are deflected, they are approximately parabolic in shape, as shown, along their length. The center portion 304 of the ribbons 302, perhaps the middle third, has a relatively flat profile, and this deflection 306 may be set to λ/4. However, the entire ribbon or diffractive element length does not have a uniform deflection. In fact, the section near the support posts 308 on either side does not deflect at all. Thus, the best optical condition can only be achieved in the middle region 304 of the ribbon 302. In practice, the optical illumination can be substantially restricted to this central region 304. This is typically referred to as the “sweet spot” or “optically active area.” In order to achieve both high efficiency and contrast, the light must be restricted to approximately the middle ⅓ of the ribbon or diffractive element length.
It would be desirable to have a light modulator design that included diffractive elements optimized to take advantage of the limited optically active areas for improved overall modulator performance.